Using the present tense for future events indicates certainty, consistency, and familiarity. In other words, use this to talk about events which will happen, which happen on a regular basis (or are predictable in some way), and about which you have some personal knowledge.
The holidays start next week.
I know this happens every year, last year I saw it happen in the same way, and I am sure it will happen the same way this year.
The train leaves in five minutes
I know the train leaves at a specific time, I know that trains usually leave on time, and I have seen the schedule to confirm when the train will leave.
The gala event starts tomorrow
I have personal knowledge that it is scheduled for tomorrow, I know that events like this usually start on schedule, and I feel comfortable asserting that it will definitely happen.
If any of these three does not exist, there is uncertainty, and the future tense will be more appropriate.
We get paid on Friday
I know this happens with some consistency and regularity, as I've already received at least one paycheck on Friday.
We will get paid on Friday
Oops, now there is some doubt. I hope this will happen, but I'm not sure it'll happen.
Here's another example: given no other context, consider these two sentences:
He will make the winning shot of the game in twenty seconds.
He makes the winning shot of the game in twenty seconds.
The first indicates a future condition, but the context is unclear. Do I really know this will happen? Probably not. More likely I'm expressing an opinion, or a personal hope.
But the second sentence, that expresses an odd certainty. The context is either that we are watching a recording of the game which I have seen before, or that I'm psychic, and I've already seen this happen with my mental powers.
Your initial examples are not as unambiguous as you think, and that is probably leading to later confusion. The continuous effect is not as baked in as you make it out to be.
"I'm having dinner at 2:00" actually sounds like dinner is scheduled to start at 2:00.
"I was having dinner at 2:00" could it was ongoing in the past at 2:00, or the 'was' could be subjunctive introducing doubt, meaning plans were changed ("I was having dinner at 2:00, but I missed the bus so we ate at 3:00"). Or even just that plans have been cast into doubt (eg "I was going to Toshi Station to pick up some power converters" -L. Skywalker)
"I will be having dinner at 2:00" could be grammatically be either ongoing or a scheduled start.
Your simple tense examples mostly work, although "I have my dinner at 2:00" sounds like it means "I have dinner at 2:00 every day" more normal usage for a specific upcoming event is "I will have dinner at 2:00"
Anyway, the main point is that the 'ongoing nature' isn't as firm a rule as you are looking for. So that's why there's nothing broken about "I'm having dinner at 2:00 tomorrow" with 2:00 being the start time.
Best Answer
Yes you can. Generally the simple present isn't used to express what is happening right now. Instead its purpose is stating general truth about things in the present. But it can also be used to describe scheduled events in the future.
Your example: "The plane arrives at 18:00 tomorrow." would fall into that category. It describes the planes arrival at a time in the future, therefore is considered a planned/scheduled event. This applies to the other examples you listed.
Edit:
I have to add that you cannot express actions in this manner, which you plan to do in the future, even if they are 'planned'. For example this -
-would be incorrect, yet:
wouldn't, since this is simply a description of the routine and could be considered a general truth.